Light-sensitive materials having a photoconductive layer which are used for making printing plates by an electrophotographic method are known. For example, light-sensitive materials used for making printing plates comprising a support having provided thereon a metal layer, a photoresist layer and a photoconductive layer have been proposed.
Conventional processes involve complicated steps such as uniformly charging a photoconductive layer, imagewise exposing it to a light to which a photoresist layer is not light-sensitive to prepare an electrostatic latent image on the photoconductive layer, developing the latent image with a toner, fixing or non-fixing a toner image, exposing the photoresist layer under a condition different from that as mentioned above to harden the exposed portion of the photoresist layer, removing the toner image and the photoconductive layer, removing the unhardened portion of the photoresist layer with a solvent, etching a metal layer and removing the photoresist layer remaining on a metal layer. All of these steps are required to prepare a printing plate using the above-mentioned light-sensitive material. Accordingly, the process requires a high degree of skill to prepare a good printing plate. In addition, the resolving power of the plate is not satisfactory since the developing method of the electrostatic latent image formed on a photoconductive layer is a dry method. Furthermore, clearness of a printing image thus-obtained is not satisfactory due to the removal of a photoresist layer and an etching operation of a metal layer.
To eliminate the above defects, European Patent Application, Published No. 0053362 discloses a light-sensitive material which comprises an electroconductive support having a hydrophilic surface, a positive working light-sensitive layer and a photoconductive insulating layer. In accordance with this structure, the positive working light-sensitive layer and the photoconductive insulating layer form a single layer or the former and the latter are provided on the support respectively. The photoconductive insulating layer is substantially capable of being charged negatively or positively and does not prevent the selective removal of the positive working light-sensitive layer. The present inventor also proposed a method for lithographic printing using the above light-sensitive material which comprises (1) a step of forming a latent image electrophotographically on a photoconductive layer of the light-sensitive material, (2) a step of developing the latent image with a liquid developer containing developer particles which are opaque to the light to which the positive working light-sensitive layer is light-sensitive, (3) a step of exposing the positive working light-sensitive layer through the developed image thus-obtained in the step (2), and (4) a step of selectively removing the area of the positive working light-sensitive layer which does not have the developed image. The above method and the light-sensitive material are disclosed and described in European Patent Application, Published No. 0053362.
The above method has advantages over the conventional method with respect to simplification of the steps and further with respect to providing satisfactory resolving power. However, a printing stain is caused when printing is carried out using the printing plate obtained by the above method in which an aluminum support or an aluminum alloy support is used as an electroconductive support.
The present inventor has investigated the cause of the stain and found the following explanation. A photoconductive insulating layer is charged and imagewise exposed to form an electrostatic latent image. The electric charge on the exposed areas is not completely removed. A toner is attached to the exposed areas during development. Where the positive working light-sensitive layer is exposed, unexposed areas remain due to the toner attached to the non-image areas as a fog in step (3). When the light-sensitive layer is selectively removed in step (4), the unexposed areas remain on the support, causing the stain. Further, the inventor has found that the electric charge is not completely removed where the photoconductive insulating layer is exposed due to the presence of aluminum oxide formed by anodic oxidation on the support.